its not good for it not something you want to do all the time but every once and awhile isnt going to grenade the tranny synchros in the trans helps minimize some of the possible damage and gears are generally pretty strong and can handle getting a little abuse so no for the most part you really are not harming the tranny that much over a life time i would say you should worry about burning your clutch out and other parts of the car just dont grind it all the time